One conventional technique for coupling an array of photoresponsive detectors to a readout integrated circuit employs indium (In) contacts or bumps between individual detectors and their associated readout circuits. The resulting hybrid photodetector/readout circuit is subsequently installed within a suitable imaging system. One conventional type of array is constructed of Group II-VI (e.g., HgCdTe) photovoltaic, infrared (IR) radiation responsive p-n diodes. In this case the hybrid photodetector/readout circuit may be installed within an evacuated container or dewar for maintaining the array at cryogenic temperatures during operation.
However, several undesirable effects have been observed when In, from the In bump which forms the electrical and mechanical bond between a HgCdTe detector and its readout circuit, comes in contact with the active HgCdTe material. First, the In has been observed to create defects in the HgCdTe. These defects cause excess leakage current to flow through the diode if the defect dislocations penetrate the p-n junction, or if they are located in the low doped, n-type side of the diode (and within a diffusion length of the p-n junction). Second, the defects provide a path through which In and Au, if Au is also used in the metal contact system, can diffuse to the p-n junction. The diffused metals can result in a short circuit across the p-n junction. Third, if In comes into contact with Au it forms a brittle intermetallic alloy which can cause the contact metal to separate from the underlying HgCdTe.
It is noted that all of these effects are accelerated when the detector is raised to elevated temperatures, as is typically the case during the vacuum bake of the dewar which houses the detector and/or during storage.
It is known to employ a layer of Ni between the In bump and the HgCdTe or a layer of Au metallization on the HgCdTe. However, the Ni interlayer has been found to not be acceptable in reliably preventing the diffusion of In into either the HgCdTe or the Au contact metallization.
U.S. Pat. No. 4,439,912 (Pollard et al.) teaches the use of a molybdenum (Mo) layer that is overcoated with an Au/Ge layer to form connecting leads to a HgCdTe epitaxial detector array that is formed on a CdTe substrate. In this application the Mo leads are said to have excellent matches to the coefficients of thermal expansion (CTE) of HgCdTe and CdTe.
Commonly assigned U.S. Pat. Nos. 5,296,384 and 5,401,986 (C. A. Cockrum et al.) teach a photoresponsive device that includes semiconductor material, such as a cap region, that is comprised of elements selected from Group IIB-VIA. A low diffusion characteristic, non-reactive refractory contact pad is formed upon a surface of the cap region. A preferred material for the contact pad is disclosed to be molybdenum. A wide bandgap semiconductor passivation or overglass layer overlies the surface of the cap region and also partially overlies the contact pad. A dielectric layer overlies the passivation layer, and an indium bump is formed upon the contact pad.
Reference may also be had "Diffusion Phenomena in Thin Films and Microelectronic Materials", Ed. by D. Gupta and P. S. Ho, Noyes Publications (1988), pgs. 432-498 for a discussion of diffusion barriers in semiconductor contact metallization. TiN is described as a well-known single phase compound diffusion barrier, and MoSi.sub.2, in combination with Al, is referred to at page 461 also for this purpose. At page 477 it is said that the silicides can be used as primary contacting layer, and not as a diffusion barrier. An example of a PtSi contacting layer with a TiN diffusion barrier is given at page 478. Table 4 lists the use of rf sputtered MoN.sub.x as a diffusion barrier in combination with a Si substrate, a Mo contacting layer, and a /Mo/Au metal overlayer.